1. Field of the Invention
This invention relates generally to apparatus and methods for cooling electronic heat generating components within a cabinet and more particularly to a method and apparatus for cooling electronic components wherein predetermined heat generating electronic components are isolated from other electronic components in the cabinet and are cooled by a dedicated air flow pathway which is isolated from ambient air in the cabinet such that the heat generated by the isolated components and non-isolated components is not mixed within the cabinet to maximize cooling of all components within the cabinet.
2. BRIEF DESCRIPTION OF THE PRIOR ART
A common problem in electronic packaging is that the heat generated by the electronic components in the cabinet is detrimental to the components themselves, particularly integrated circuits and microprocessor chips in computer cabinets. Heat is normally removed by circulating air across the components by one or more low powered exhaust fans mounted in or on the computer cabinet.
The microprocessor chip in a computer generates a relatively large amount of heat and is susceptible to error or damage caused by overheating. For example, the Intel 80486 microprocessor chip generates 4.5 watts in normal operation and must be maintained below 85 degrees C. or it can introduce error as well as reduce its operating life. Even newer chips will generate 15-30 watts. To assist heat removal from an integrated circuit chip, a heat sink is often mounted on the top surface of the chip. Heat sinks are metal devices that have a plurality of fins or pins extending from a base which is mounted on the chip surface to transmit heat from the chip to the circulating air. The fins are usually aligned longitudinally with the prevalent direction of air flow and when the air flow direction is uncertain, pin-type heat sinks may be used.
The resistance to heat transfer from the hot surfaces to the air is provided primarily by the thin laminar boundary layers that grow on surfaces as the air flows along them. To reduce the thickness of these boundary layers, and hence their thermal resistance, the conventional approach is to reduce the length of the fins in the direction of air flow or to stagger them so the laminar boundary layers are broken and begin to grow from the leading edge of each cut or staggered fin. Heat sinks built with cut or staggered fins are far more expensive than heat sinks with uninterrupted fins, because the former requires a complicated machining or molding process and the latter can be made by a simple extrusion process.
The exhaust fan in a computer cabinet usually develops a single air flow path wherein air from the cabinet exterior is drawn in through inlet slots in the cabinet wall flows across the components including the heat sink and is exhausted through exhaust slots in the fan motor housing in another wall of the cabinet. Although the heat sink is useful in cooling the microprocessor chip, the heat generated by the heat sink is mixed with the heat generated by the other components in the cabinet such that the effective cooling of all the components including the ones having heat sinks is diminished.
In some computer cabinets, such as laptop and notebook computers, there is no fan in the cabinet or case, because the fan presents a relatively large drain on their batteries. CPUs now in laptop and notebook computers, especially those for 80486 chips, must be run more slowly to avoid over heating. Many laptop and notebook computers are built with two speeds: (1) slow speed for operating on battery power, and (2) high speed for plugging into an electrical outlet. However, in the latter case there is danger that the chip will overheat.
Because of the compact space requirements there is often insufficient space in some laptop and notebook computers to utilize components with finned heat sinks. Some manufacturers of notebook and laptop computers provide an aluminum heat spreader 1/16" to 1/8" thick between a pair of circuit boards that extends across almost the entire computer cabinet. The purpose of the heat spreader is to conduct heat to it's periphery, where it can be radiated to the case (usually plastic) and then delivered to the environment by free convection. Obviously, this path has a high thermal resistance, and a powerful high frequency 80486 chip may still overheat if running full speed.
Japanese patent 56-148898 shows a circuit board that has a divider with raised walls with some components having a heat sink disposed inside the walls and some components outside the walls. The board is installed in a slot in the computer chassis and air is drawn by the fan through the slots and through an angled inlet of the divider. The divider wall elements are no taller than the fins of the heat sink and are positioned only on two sides. Thus, the divider does not enclose the heat generating components, and the air passing through the divider elements becomes heated by the first components and more heat is added by the successive heat generating components as the air travels downstream and is less effective in cooling the downstream components. This heated air will also pass over the divider walls and mix with the air drawn through the slots and will heat the air passing over the other components and throughout the cabinet such that effective cooling of all the components in the cabinet is diminished.
An IBM Technical Disclosure Bulletin, Volume 19, No. 12, May 1977 discloses a method of fitting a heat generating module with an air shroud. The air shroud has an inlet port connecting to a pressurized air line and an exhaust port which just exhausts air to the exterior of the shroud into the cabinet. The high pressure, high velocity air is directed over the top surface and over the underside of the heat generating module (the chip, substrate, and can). The heat generating chip and substrate are enclosed by the can which serves as a heat sink and together they form the heat generating module. These components are enclosed in the shroud. The hot air inside the can passes through the pins where it is transferred to the card column air flow in the cabinet. The air inside the shroud passing over the top surface and over the underside of the heat generating module is heated by the module, and this heated air is exhausted through the exhaust port of the shroud into the cabinet where it would be mixed with the heated air of the card column air flow such that effective cooling of all the components in the cabinet is diminished.
Others have attempted to solve the heating problem by mounting a small supplementary fan on the heat sink, and mounting liquid cooling devices (water jackets) or devices utilizing the Peltier effect on the chip to be cooled.
The present invention overcomes the heating problem by enclosing and isolating predetermined heat generating electronic components which may include those with heat sinks from other electronic components in the cabinet and providing a dedicated air flow pathway to conduct a stream of air from the cabinet exterior, across the isolated electronic components, and exhaust it to the cabinet exterior. The dedicated air flow is isolated from the ambient air in the cabinet such that the isolated heat generating components are cooled independently of the non-isolated components and the heat generated by the isolated components and non-isolated components is not mixed within the cabinet to maximize cooling of all components within the cabinet. Cooling efficiency is increased by creating turbulence in the dedicated stream of air which passes across the isolated heat generating component or the fins of the heat sink before being exhausted and by providing specially designed inlet and outlet conduits to improve the flow rate and to reduce the pressure drop across the heat generating electronic component.